In the art of liquid cooled brake units for stopping the rotation of a shaft, for example, of the type disclosed in U.S. Pat. No. 4,415,067 which issued to the Assignee of the present invention, it is sometimes desirable to provide for operating the brake unit with electrically actuating means to avoid extending an air supply line from an air compressor or an air supply system. Also, some air supply systems pick up dirt and moisture which can require more frequent servicing of an air actuated brake unitl it is further desirable for an electrically actuated brake unit to provide for simplified and economical construction which also provides for dependable operation over an extended period of of use in addition to simplified assembly and servicing. Furthermore, in some installations, it is desirable for an electrically actuated brake unit to provide for stopping a shaft with selectively different braking torques. For example, when an electrically actuated brake is used on the slide drive for a machine tool carriage or table, a high torque may be used to stop the slide drive motor and a low torque may be used to produce a predetermined thrust or force of the slide against a positive stop during a machining operation.
One form of oil-cooled electromagnetic brake is disclosed in U.S. Pat. No. 2,884,107. In this patent, a large annular magnetic coil and an annular armature plate surround or are concentric with the shaft which requires braking. Another form of dry-type electrically actuated brake is disclosed in U.S. Pat. No. 2,025,098. In the embodiment shown in FIGS. 3 and 4 of this patent, a set of circumferentially spaced solenoids have cylindrical armatures pivotally connected to pins which support an annular thrust plate located between the solenoids and the stack of brake plates and discs. The braking operation is produced by a set of circumferentially spaced compression springs, and the braking operation is released by retraction of the annular thrust plate with the connecting rods when the solenoid coils are energized.